Oven for heating moving thermoplastic material container blanks

ABSTRACT

The invention concerns an oven ( 1 ) for heating moving thermoplastic blanks (preforms, intermediate containers), comprising conveying means ( 2 ) supporting and moving the blanks sequentially while rotating each of them about its own axis and heating means ( 3 ) arranged laterally to said conveying means ( 2 ) for heating the moving blank bodies; the conveying means ( 2 ) include at least two parallel conveying branches ( 4, 5 ), proximate each other and having opposite conveying directions, said two branches being travelled along one after the other by the blanks; the heating means ( 3 ) are arranged between said two conveying branches ( 4, 5 ) so as to heat simultaneously on both sides the blanks moving in opposite directions respectively on said two branches.

The present invention relates in general to the field of the manufactureof containers, such as bottles, flasks, etc., made of a thermoplastic,such as PET or PEN, by blow molding or stretch-blow molding startingcontainers or blanks (preforms or intermediate containers) and, morespecifically, the invention relates to the phase during which saidblanks are heated prior to the blow molding or stretch-blow moldingstep, so as to soften the plastic and make it deformable.

More precisely, the invention relates to improvements made to ovens forheating blanks on the run, especially preforms or intermediatecontainers, made of a thermoplastic, this oven including conveyingmeans, suitable for supporting and moving the blanks one after anotherwhile rotating each of them about its own axis, and heating means placedlaterally to the conveying means so as to heat the bodies of said movingblanks.

So that the stretching process takes place correctly and uniformly overthe entire body of the blank, it is necessary for all of thethermoplastic to be raised to a sufficient temperature to be softenedwithout, however, this temperature being excessive in order to preventthe material from crystallizing. This means that, despite the poorconductivity of the thermoplastic, a sufficient amount of heat(generated for example continuously by infrared radiation) propagatesthrough the entire thickness of the wall of the blank (e.g. heating tothe core).

In the case of certain heating means, such as infrared lamps, no purposeis served by forcing the infrared transmission power in order to try toachieve this result more quickly, since the poor thermal conductivity ofthe thermoplastic entails a certain lapse in time in order to achievecore heating, and a reduction in this time, by increasing the power ofthe infrared radiation, runs the risk of causing the surface material onthe external face of the wall to crystallize. Such a solution musttherefore necessarily be discarded.

A solution to the problem must therefore be sought in an extension ofthe time during which the thermoplastic is exposed to the heating means.

One conceivable solution might consist in slowing down the blanks thatare moving past the laterally positioned heating means. However, itshould also be noted that container manufacturers are constantly seekingto increase production rates, which can be achieved in particular byincreasing the machine speeds. Such an aspiration therefore goes counterto lowering the speed of movement of the manufacturing line at any pointthereon. This solution must therefore be discarded also.

Admittedly, it would be conceivable to reduce the run speed of theblanks through the oven (for example reducing the speed by half), buthaving two ovens fed in parallel—the total treatment capacity would thenbe maintained. However, this solution would be very expensive both interms of equipment and in energy, and must also be discarded.

A useful solution might consist in increasing the length of the oven,which, for a given run speed of the blanks, would lead to an increase inthe exposure time. It then becomes conceivable to reduce thetransmission power of the heating means, this being permitted by thefact that there is then enough time for the heat to propagate throughthe thickness of the wall of the blanks. Admittedly, this solutionentails an overcost in terms of equipment, in order to lengthen theoven, but in use it proves to be economic owing to the reduction (whichmay be up to 35%) in the energy needed for the heat treatment of theblanks.

However, this solution, although generally beneficial for the reasonsindicated above, has a drawback due to the great length of the oven thusprovided, which proves to be difficult to implement in practice.

It is under these conditions that the invention proposes an improvedsolution, which meets the requirement expressed by longer exposure ofthe blanks to the heating means while still avoiding the variousdisadvantages of the abovementioned solutions.

For this purpose, the invention proposes an oven for heating blanks onthe run, especially preforms or intermediate containers, made of athermoplastic, this oven comprising conveying means, suitable forsupporting and moving the blanks one after another while making each ofthem rotate about its own axis, and heating means placed laterally tothe conveying means so as to heat the bodies of said moving blanks, saidoven, being designed in accordance with the invention, is characterizedin that the conveying means are arranged so as to have at least twoconveying branches lying approximately parallel to each other, and neareach other, and having opposite conveying directions, the blankstraveling along said two branches one after each other, and the heatingmeans being placed between said two parallel conveying branches andarranged so as to heat, bilaterally and simultaneously, the blanksrunning in opposite directions along the two conveying branchesrespectively.

Preferably, the two branches are joined, at one of their ends, by a loopconveying section that is located outside the zone where the heatingmeans act and which consequently constitutes a zone for thermallystabilizing the blanks.

Thanks to this arrangement, each blank passes at least twice through theoven, with an interruption between these two heating phases, whichcorresponds to the path in which the conveying means turn around andwhich constitutes a stabilization step during which the heat continuesto diffuse into the material. The heating time is thus doubled, andresults in a thermoplastic being heat treated under optimum conditions,causing the entire plastic to be heated uniformly and right to the core.

From the structural standpoint, the aforementioned beneficial result isobtained with a single combination of heating means, the effectivenessof which is doubled by the fact that use is made here of their radiationin two directions, and no longer in one direction as in a conventionaloven.

Admittedly, oven arrangements with two files of moving objects to betreated on either side of heating means are known (see for exampledocuments EP-A-0 868 284 and DE-A-24 27 611). However, in these knownarrangements, the two files of objects to be heated move in the samedirection. These known ovens make it possible to double the treatmentcapacity, but each object performs only a single pass through the oven.Consequently, the exposure time of each object is not extended relativeto the exposure time of an object passing through a conventional oven(i.e. a single file of objects) of the same length.

In addition, it should be noted that, in an oven with two passes inopposite directions, as recommended in accordance with the invention,the two successive exposures of each object to the heating means,resulting in an increase in the exposure time, may advantageously beaccompanied by a reduction in the power of the radiation emitted by saidheating means, and therefore by a reduction in the electrical powerconsumed. This is particularly of interest to the user (trials havedemonstrated that the saving obtained, using infrared lamps, amounts to,or even exceeds, 50%).

In one advantageous embodiment, the oven includes two pairs of parallelconveying branches with heating means placed between the two branches ofeach pair respectively, the four conveying branches being connectedtogether via loop conveying sections located outside the zones where theheating means act. To make a compact oven, it is advantageous for thefour conveying branches to be mutually parallel and preferably, for thebranches to be approximately rectilinear.

Preferably, reflectors may be associated, in a manner known per se, withthe heating means, these reflectors being placed alongside eachconveying branch on the opposite side from that occupied by the heatingmeans.

The invention will be more clearly understood on reading the followingdetailed description of certain embodiments given as entirelynon-limiting examples. In this description, reference will be made tothe appended drawings in which:

FIG. 1 is a simplified diagram illustrating an oven arrangementimplementing the provisions of the invention; and

FIG. 2 is a simplified diagram illustrating an advantageous example ofoven embodiment exploiting the provisions of FIG. 1.

Referring firstly to FIG. 1, an oven 1 is intended for heating blanks onthe run, especially preforms or intermediate containers, made of athermoplastic such as PET or PEN, it being possible for such an oven tobe placed in particular upstream of a blow molding or stretch-blowmolding installation for manufacturing containers.

The oven 1 includes conveying means 2 suitable for supporting and movingblanks (not shown) one after another, while rotating each of them aboutits own axis. These conveying means may consist, in a manner known perse, of an articulated sequence or chain of support devices (“spinners”)suitable for supporting the blanks by their neck, while at the same timethese support devices are rotated (for example by the meshing of atoothed wheel, which is fastened thereto, with a fixed lateral chain orrack) in such a way that each blank, at the same time as it is beingmoved, is rotated so that its body is exposed over its entire peripheryto heating means 3 placed laterally to the conveying means.

According to the invention, the conveying means 2 are designed so thatat least two conveying branches 4, 5 lie approximately parallel to eachother and near each other, while having opposite directions of movement.The blanks travel, one after another (in series), as shown symbolicallyby the arrows 6, along the two branches 4, 5.

The heating means 3 are placed between the two branches 4, 5 of theconveying means 2 and are designed so as to operate on both sides and tosimultaneously heat the blanks moving in opposite directions along thetwo branches 4, 5 of the conveying means, respectively.

As an example, FIG. 1 shows the heating means 3, which here are in theform of three successive sets 7 of infrared heating lamps.

The two branches 4, 5 of the conveying means, which in a preferredembodiment are approximately rectilinear inside the oven as illustratedin FIG. 1, are simply joined to each other, at one end (on the right inFIG. 1) of the oven 1, by a loop-shaped section 8 of the conveyingmeans.

The loop section 8 of the conveying means, apart from its function ofjoining together the branches 4, 5 that pass through the oven, also actsas a stabilization zone, allowing the heat to continue to propagate intothe thermoplastic and preventing excessive heating of the external faceof the body of the blanks.

The oven 1 is equipped in a manner known per se with reflectors 9 placedfacing the heating means 3, on the other side of the branches 4, 5 ofthe conveying means.

In an oven designed according to the invention, each blank passes twicein succession opposite the heating means with an intermediate thermalstabilization period. For an oven of given length, the same heating timeis obtained for each preform, with a substantial reduction in the powerof the heating means and therefore in the electrical power consumed,without an increase in the number of heating lamps and withoutincreasing the working length of the oven, compared with a conventionaloven having a single conveying file, although the preforms are heatedmore uniformly and to the core.

FIG. 2 illustrates schematically a specific example of an ovenimplementing the above provisions. In practice, the arrangementillustrated in FIG. 1 consists in duplicating (with parts A and B) thearrangement shown in FIG. 1, the blanks then making four successivepasses through the oven, and therefore there are four successiveheatings, interspersed with thermal stabilization periods.

The blanks arriving at E are delivered by means 10 (such as a feedwheel) for charging the conveying means 2, consisting of a closed-loopchain 2 of support devices. The chain 2 then enters the part A of theoven 1 via its branch 4A, where the blanks undergo a first heatingoperation, and then, after passing along the loop section 8A, a secondheating operation on the branch 5A.

The blanks then pass along a joining section 11, which also defines athermal stabilization zone, bringing them to the second part B of theoven, in which they follow a similar path: branch 4B with heating, loopsection 8B with thermal stabilization, branch 5B with heating.

On leaving the branch 5B, a section 12 brings the blanks to theunloading means 13 (such as a “discharging” wheel) where they aredetached from the support devices of the chain 2 (which chain returns tothe charging means 10) and are discharged at S toward, for example, ablow molding or stretch-blow molding unit. Since the discharging means13 are not immediately adjacent to the exit of the oven, the section 12itself constitutes a thermal stabilization zone that completes thediffusion of heat into the thermoplastic.

The four branches 4A, 5A, 4B, 5B may be mutually parallel so that it ispossible to construct an oven of simple and compact design, with littleheat loss. Furthermore, it is possible to place, in the central part,between the abovementioned parts A and B of the oven, members that arecommon to the operation of these two parts A and B, and especially a fandelivering the gas that cools the surface of the blanks.

1. An oven for heating blanks on the run, especially preforms orintermediate containers, made of a thermoplastic, this oven comprisingconveying means, suitable for supporting and moving the blanks one afteranother while making each of them rotate about its own axis, and heatingmeans placed laterally to the conveying means so as to heat the bodiesof said moving blanks, wherein the conveying means are arranged so as tohave at least two conveying branches lying approximately parallel toeach other, and near each other, and having opposite conveyingdirections, the blanks traveling along said two branches one after eachother, and the heating means being placed between said two parallelconveying branches and arranged so as to heat, bilaterally andsimultaneously, the blanks running in opposite directions along the twoconveying branches respectively.
 2. The oven as claimed in claim 1,wherein the two conveying branches are joined, at one of their ends, bya loop conveying section that is located outside the zone where theheating means act.
 3. The oven as claimed in claim 1, wherein itincludes two pairs of parallel conveying branches with heating meansplaced between the two branches of each pair respectively, the fourconveying branches being connected together via loop conveying sectionslocated outside the zones where the heating means act.
 4. The oven asclaimed in claim 3, wherein the four conveying branches are mutuallyparallel.
 5. The oven as claimed in claim 1, characterized in that theconveying branches are approximately rectilinear.
 6. The oven as claimedin claim 1, wherein reflectors are placed alongside each conveyingbranch on the opposite side from that occupied by the heating means. 7.The oven as claimed in claim 1, wherein the heating means compriseinfrared lamps.